When Katherine Jeffrey told her Oxford tutor she wanted to be a teacher, he was shocked. It wasn’t what he was used to hearing from his politics, philosophy and economics undergraduates. “He couldn’t remember the last time one of his students had wanted to teach,” she says. “During the first two years at Oxford I considered journalism, or glamorous jobs in the City. In my final year I decided I still wanted to do what I’d wanted to do since I was five - teach.”

Now 32, Ms Jeffrey is the new head of the oldest girls’ Catholic boarding school in Britain, New Hall school in Chelmsford, Essex. It’s a defiant appointment from one of education’s most rapidly shrinking sectors. Seventy-five per cent of independent Catholic schools have disappeared in the past 40 years, leaving around 140.

Ms Jeffrey is assured, utterly at home in her new role in the former Tudor mansion, acquired by Henry VIII from his father-in-law, Sir Thomas Boleyn, with its parkland and chapel. And she is fast becoming the UK’s most articulate and outspoken champion of girls’ education. There is no defensiveness as she outlines her beliefs. The school’s former librarian has been appointed its full-time public relations officer, and Ms Jeffrey gives every sign of breathing deeply the oxygen of publicity. She is, of course, a good story: the school’s first lay headteacher; one of the UK’s youngest heads; a mother of two whose younger child was six weeks old when she started the job in the Easter term.

“What a role model for girls,” is the PR line, but what exactly is that role - having it all, or having to do it all? The longing for children is a reality for many women who have put career before conception, and offices are full of mothers who dare not put their children’s photographs on their desks. A new headship six weeks after giving birth is, by anyone’s standards, going it a bit.

“There’s never a right time to have a baby. I would not want our girls to feel they have to be superwomen. You can’t do it all. Something has to give,” says Ms Jeffrey, whose family lives in the on-site school house, where a live-in nanny looks after Rebecca, now eight months old, and four-year-old Anna.

“We can’t run away from a tension here: if you educate girls to a certain level, chances are many will not be happy at home raising children for years on end. I could not do it. There’s a limit to how long I can run round the garden playing peek-a-boo. I adore my children and they are my priority, but I did find being at home frustrating. Most young girls today will want some sort of career and a family. There’s a balance to be redressed and that can only be done by educating boys to take on their share of family life.

“Some of my friends with jobs in the City have found their careers incompatible with family life, and insufficiently fulfilling and rewarding. Quite a lot of my peers have come into teaching late. There’s stress in teaching, but the job is still compatible with family.

“Many girls are attracted to teaching. When a pupil tells me she wants to teach, I’m always delighted. She often says I’m the first person she has told who has enthused about it.

“There’s a glass ceiling in the City, not so much because men prevent women from getting to the top, but because of the nature of the work. Many women don’t want everything that goes with it: the long hours, people being treated poorly, the absolute priority the job has if you take a partnership in a law firm.”

No doubt she easily could have done just that herself. In fact, with her chic navy suits and short hair, her confidence and academic background, she could pass for one of Tony Blair’s young Turks, a schools minister rather than the RE teacher she became.

Ms Jeffrey has a lifestyle she loves. She plays Debussy on the piano in the evenings. And she’s up at 6.30 every morning to run, sometimes making the six-mile round trip between school and station to see her lawyer husband, Sean, off on his daily trip to London.

Running is part of her routine - and of her role-modelling. “I started running at university with my husband. He said it was the only time I ever listened to him, because I was too breathless to talk. Now exercise is important to me. It’s a good example for the girls too.”

Ms Jeffrey chose to teach religious education largely because of its philosophical content. A lifelong Catholic (her brother is training for the priesthood), she started her career at St Mary’s school in Ascot, Berkshire. She spent six years doing a theology degree by distance learning and took an MA in educational management. Before coming to the 660-pupil New Hall, she spent a year as deputy of the Marist Convent school, also in Berkshire.

Convent schools are surrounded by cliche and stereotype. A lay head is certainly likely to make schools such as New Hall more attractive. The Year 12 entry there has risen from 47 last year to 67 this September. It makes a change from the assumption that girls-only education is on the decline.

“I’m an unashamed fan of single-sex senior education for girls. Boys can thrive equally in a co-ed or in a single-sex school. At secondary level, girls’ schools cater better for their needs,” says Ms Jeffrey, who has clearly had some practice in this argument. “Of course, some people say it isn’t right - that secondary education should be an education for life. I say you only go through adolescence once.

“The obvious distractions in a mixed school can be fatal at an early stage. There’s tremendous pressure on young girls to value themselves for what they look like rather than what they do. In a co-ed, being successful is seen as unfeminine and they may hide their talents; in a single-sex school they are more forward about their academic ability and they develop greater self-esteem.

“Boys tend to claim the air space. We’ve all seen the studies showing that boys can claim 80 per cent of a teacher’s attention in a co-ed class. In a single-sex school girls are encouraged to stretch themselves, to be heard and to fill leadership roles. You see girls who come in very quiet at 11 simply blossoming.”

Before Katherine Jeffrey was old enough to go to school, my headmistress used to quote to her young blossoms the line from the beginning of Desiderata: “Go placidly amid the noise and haste and remember what peace there may be in silence.” There is little peace in education today; it is becoming increasingly skills and jobs-oriented, and for many young people life is about the power to buy.

How does the fast-running, high-flying head of New Hall feel about peace? “As a Catholic school we have a prayer life, and that gives a greater understanding of God and spirituality. We have a community of 20 nuns whose lives revolve around prayer. That gives a tremendous sense of peace and contemplation.

“That can’t be bolted on to the curriculum, as if it were citizenship. You have to live and experience spirituality; it’s not something that comes off a shelf from a shopping centre. You can’t simply say, ‘Right, girls, now let’s do some spirituality.’ Religious education is important. It’s about grappling with the big questions: why are we here? How can you be good? Is there a God and can we prove it?

“We are not just another girls’ school getting our pupils through exams. Our girls can go and take the world on rather than be changed by it. You can never protect your child from the world but you can equip her for it with confidence, integrity and good judgment.”

The community of nuns, who turned New Hall into a school in 1798, live in one end of the building and five of their members teach at New Hall.

The world is ever with us, even those of us who have a sisterhood of nuns on the other side of the wall and figures of the Virgin Mary in the corridors. Katherine Jeffrey stands in the front line, facing the world on behalf of Catholic schools and girls’ education. And she will be heard.